Forced.
15... d7b6 The most direct move, preparing to capture on c4 and also Na4. 16. c3a5 White pins the knight, and play becomes concrete, with possible exchanges in the center. 16... d5c4  17. d2c4  17... b7f3 A dubious decision that allows White to keep a small advantage, thanks to the power of the famous \"Catalan Bishop.\"